1. Field of the Invention
The invention relates to a semiconductor light-receiving device to be employed for optical communication and optical data processing, and a method of fabricating the same.
2. Description of the Related Art
A compound semiconductor light-receiving device has been put to practical use as a wide-range wavelength light receiver with high sensitivity for optical communication and optical data processing. Above all, a semiconductor light-receiving device for a wavelength of 1.3 .mu.m or 1.55 .mu.m, which is a wavelength for high capacity long-distance optical communication, is usually made of InGaAs.
It is necessary for a PIN photo diode made of InGaAs to minimize a junction capacity of individual devices in order to accomplish ultra-high speed response greater than 40 Gbps, and to form a light-absorbing layer thinner in order to reduce running time of carriers.
However, in a presently commercially available light-receiving device having a surface through which the device receives a light, if a light-receiving diameter is made smaller for reducing a junction capacity, optical coupling would be difficult to properly take place when the light-receiving device receives a light from an optical fiber, resulting in deterioration of coupling efficiency. On the other hand, if a light-receiving layer made of InGaAs were formed thinner, it would be possible to reduce running time of carriers, however, with the result of reduction in quantum efficiency. Thus, reduction in a junction capacity and enhancement in coupling efficiency, and reduction in carrier running time and enhancement in quantum efficiency are in a trade-off relationship. Hence, since a direction in which a light is directed is coaxial with a direction in which carriers run in either process, reduction in light-receiving sensitivity is unavoidable, even if high-speed response could be accomplished.
In order to avoid influence due to the above mentioned trade-off relationship, there has been suggested a waveguide type light-receiving device in which a direction in which a light runs is deflected by 90 degrees to a direction in which carriers run.
FIG. 1 illustrates one of conventional waveguide type light-receiving devices, which has been suggested by K. Kato et al. in "High efficiency, waveguide InGaAs pin photodiode with bandwidth of 40 GHz", Electronic Data Communication Association Spring Conference, 1991, pp. 4-200, C-183. The illustrated waveguide type light-receiving device includes a semi-insulating InP substrate 20, an insulating layer 21 formed on the substrate 20, a waveguide type light-receiving region 22 formed on the insulating layer 21, a pair of polyimide layers 23 sandwiching the light-receiving region 22 therebetween, a p-side electrode 24 formed over the light-receiving region 22 and the polyimide layers 23, and an n-side electrode 25 formed on the insulating layer 21. The waveguide type light-receiving region 22 acts as a waveguide, and is comprised of an n-InGaAsP clad layer 22a, an n-InGaAs light-absorbing layer 22b, a p-InGaAs light-receiving layer 22c, a p-InGaAsP clad layer 22d, and a p-InGaAsP contact layer 22e. The waveguide type light-receiving region 22 is formed by successive, epitaxial growth of these layers. The p-side electrode 24 makes electrical contact with the p-InGaAsP contact layer 22e, and the n-side electrode 25 makes electrical contact with the n-InGaAsP clad layer 22a. The polyimide layers 23 sandwiching the light-receiving region 22 therebetween to thereby reduce a capacity of a bonding region.
In the illustrated waveguide type light-receiving device, a light is introduced into the device through an end surface of the light-receiving region 22, and is transferred and absorbed in the n-InGaAs light-absorbing layer 22b and the p-InGaAs light-absorbing layer 22c both vertically sandwiched between the clad layers 22d and 22a.
Carriers generated due to absorption of a light are transferred to an external circuit (not illustrated) through the p-InGaAsP clad layer 22d, p-InGaAsP contact layer 22e, and p-side electrode 24. Since absorption of a light is accomplished in a length-wise direction of the light-receiving region 22, it is possible to obtain high quantum efficiency. In addition, since carriers run perpendicularly to the light-receiving region 22, running time of carriers is dependent only on a thickness of the n-InGaAs light absorbing layer 22b.
The above mentioned waveguide type light-receiving device has a problem as follows. If the n-InGaAs light absorbing layer 22b is formed thinner for reducing carriers running time, optical coupling efficiency would be deteriorated because a light introduced into the device through an optical fiber has a greater diameter than a thickness of the n-InGaAs light absorbing layer 22b. In contrast, if the n-InGaAs light absorbing layer 22b is formed thicker for enhancing optical coupling efficiency, it would be accompanied with an increase in running time of carriers.
FIGS. 2A to 2C illustrate another semiconductor waveguide light-receiving device suggested in Japanese Unexamined Patent Publication No. 3-35555. FIG. 2A is a top plan view of the device, FIG. 2B is a cross-sectional view of the device taken along the line IIB--IIB in FIG. 2A, and FIG. 2C is a cross-sectional view of the device taken along the line IIC--IIC in FIG. 2A.
The illustrated semiconductor waveguide light-receiving device includes a semiconductor substrate 30, a buffer layer 31 formed on the substrate 30, a waveguide 32 formed partially on the buffer layer 31, a light-receiving layer 33, and a junction forming layer 34. The waveguide 32, the light-receiving layer 33, and the junction forming layer 34 are sandwiched by fillers 35. The light-receiving layer 33 and the junction forming layer 34 cooperate with each other to form a light-receiving device, and are formed tapered in a direction indicated with an arrow X in which a light is directed. On the junction forming layer 34 are alternately formed p-type regions 36a and n-type regions 36b arranged in a direction indicated with the arrow X.
A light is introduced into the device through an end surface 37 thereof, and is transferred through the waveguide 33 in a direction indicated with the arrow X.
FIG. 3 illustrates still another semiconductor waveguide light-receiving device suggested in Japanese Unexamined Patent Publication No. 4-268770.
The illustrated device includes an InP substrate 40, an InGaAsP clad layer 41 formed on the substrate 40, an InGaAsP core layer 42 formed on the InGaAsP clad layer 41, clad layers 43 and 44 both made of an InGaAsP multi-layered structure formed on the InGaAsP core layer 42, an InGaAsP layer 45 formed on the InGaAsP core layer 42, an n-side electrode 46 formed on a lower surface of the substrate 40, and a p-side electrode 47 formed on an upper surface of the InGaAsP layer 45. The multi-layered structure 43 and 44 are formed by alternately depositing layers for absorbing a light thereinto and layers transparent to a light. The multi-layered structure 44 is formed partially on the multi-layered structure 43 in the form of a ridge.
The multi-layered structures 43 and 44 act as a light-absorber. A light is introduced into the device through a side surface thereof in a direction indicated with an arrow X, and is converted into electricity in the multi-layered structures 43 and 44.
In both of the above mentioned waveguide light-receiving devices, a light is introduced into the devices through end surfaces thereof. However, the devices have a problem that optical coupling efficiency is quite low. In the above mentioned conventional waveguide light-receiving devices, if an n-InGaAs light-receiving layer is formed thinner for reducing running time of carriers, optical coupling efficiency would be deteriorated because an incident light has a sufficiently greater diameter than a thickness of the n-InGaAs light-receiving layer. In contrast, if an n-InGaAs light-receiving layer were formed thicker for enhancing optical coupling efficiency, running time of carriers would be increased. Thus, the above mentioned waveguide light-receiving devices could not improve optical coupling efficiency.